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PostPosted: Sat Jul 28, 2018 1:21 pm 
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Hi All
Wonder if anyone has ever come across this issue before :-
Baxi 105 he combi boiler
The boiler is approx. 10yrs old but ALL the radiators are approx. 3yrs old and the system pipe work is about 80% plastic 20% copper also 3yrs old the system has a Fernox TF1 filter/mag on

The pressure on the gauge on the boiler is INCREASING slowly
I first noticed that the pressure gauge had gone up to 1.5bar (it normally runs at 1bar) but thought it was with maybe having the heating on or running the bath so just monitored it

It did NOT return to normal (1 bar) when the system was completely cold and left overnight
I thought it was maybe the filling loop by passing - so replaced the fill valve dropped the pressure and disconnected the filling loop hose

But I have noticed that the pressure is slowly going UP again !!

I dropped the pressure again and since then have not used the CH or HW as an experiment to see if the pressure would still increase - which it is - Im talking from dead on the 1 bar mark to half way across the figure 1 in about 3-4 weeks
Not a lot I know but if there is an issue I would like to get it fixed before the boiler is needed in the autumn
The CH system is working fine and so is the HW system
I did put Inhibitor in the system via the Fernox TF1 filter approx. 4 months ago

I have been doing a bit research on here and on the net and have come up with the following :-
1. Pin hole in the plate heat exchanger allowing mains pressure into CH system
2. Expansion vessel - ( though pressure normally DROPS if its broken)
3. Chemical reaction in the CH system producing gas / air - thus increasing pressure !!!

Do any of these seem to make sense ??
Any help advice much appreciated
Thanks in advance

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PostPosted: Sat Jul 28, 2018 4:39 pm 
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Is there an old filling loop not used anymore but left connected somewhere that could be letting by?

Other than that as long as you have done what you have posted properly, can only be a pin holed plate.

Pressure boiler to 1bar exactly, turn off cold mains inlet valve under boiler and leave off for as long as possible.

If pressure doesn’t rise it proves plate pin holed.


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PostPosted: Sat Jul 28, 2018 6:18 pm 
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Hi Dave
Thanks for the reply
My first thought was the filling loop so dropped the pressure & disconnected the filling loop so I could eliminate that
When you say the mains inlet - I assume that you mean - what would run through the boiler and supply the HOT WATER tap ??
To be honest I had not thought of that - so THANK YOU for that tip
I will do as you recommend as I can live without the HOT water as I have an electric shower so it is only used for washing dishes etc
It is a VERY slow leak so might take a couple of weeks to post back the results but
Thank you for you time & help

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